Free cortisol method comparison: ultrafiltation, equilibrium dialysis, tracer dilution, tandem mass spectrometry and calculated free cortisol.

Background: Free cortisol (FC) can be calculated from measurements of total cortisol and binding proteins or measured after mechanical separation of unbound and bound fractions by equilibrium dialysis or ultrafiltration. FC can then be measured indirectly by 3H-cortisol dilution or directly by immunologic or tandem mass spectrometry assays.

Methods: We compared FC measured with ultra high performance liquid chromatography-tandem mass spectrometry (UHPLC MS/MS) with 3H-cortisol dilution in ultrafiltrates and dialysates and also with calculated FC (Coolens equation).